Exemplary embodiments of the invention relate to a thermal bridge element, particularly for a space flight instrument, a satellite, a means of transportation, or a machine component, and to a method for producing the same.
Thermal bridge elements are used for the thermal coupling of two components for heat transfer. Frequently, the bridge elements have a flexible design to allow a relative movement of the components connected to each other by the bridge element.
Heat transport by means of solids is of particular importance in space flight applications, like a space flight instrument or a satellite, for example, because due to the absent medium air, the heat transport can primarily take place by means of solids, radiation, or multi-phase systems. In the field of transportation means, such as vehicles, for example, or in machine construction, such requirements are also found.
As a rule, flexible, thermal bridge elements are realized from metals, like, for example, copper or aluminum. For this purpose, multi-layered strips of metal foils can be produced and interconnected, for example, in order to be able to provide flexibility about an axis. At the same time, a sufficiently large cross-sectional surface is provided to ensure the heat transport by way of such a bridge element. The advantage of such connections is the simple processability of metals. The disadvantage is a relatively high specific weight in relation to conductivity. While copper has higher heat conductivity as compared to aluminum, the density is greater at the same time. A higher weight resulting therefrom is of disadvantage, particularly in space flight applications.
Furthermore, thermally highly conductive carbon fibers are known, the guide values of which in the fiber direction are, in relation to their thermal conductivity, three to four times as high as aluminum. Such fibers can transport more heat at significantly less weight. However, the production of a thermal, flexible bridge element is difficult due to the brittleness of the individual carbon fibers. For example, a strong clamping of the brittle carbon fibers, which is necessary for good heat transfer between the bridge element and the components coupled to the bridge element, results in substantial damage to at least some carbon fibers so that they can no longer contribute to heat conduction. Because impurities can also materialize from damage to carbon fibers, which are particularly problematic in space flight applications, bridge elements consisting of carbon fibers have not yet reached production line status.
Exemplary embodiments of the present invention are directed to a thermal bridge element, the weight-specific heat conductivity of which is significantly higher than in a component made of conventional materials. Furthermore, a method for producing such a thermal bridge element is to be disclosed.
In accordance with exemplary embodiments of the present invention a thermal bridge element is provided, in particular for use in a space flight instrument, a satellite, a transportation means (a motor vehicle, for example), or a machine component. The bridge element comprises a number of carbon fiber-reinforced plastic layers (thereafter: CFK layers), wherein each of the CFK layers is composed of a plurality of heat-conductive carbon fibers embedded in a matrix. On at least two segments, in particular opposing end segments, the CFK layers are freed of the material of the matrix so that the carbon fibers of a respective CFK layer in the at least two segments are exposed. The exposed segments of the carbon fibers are provided with an associated metallization, by way of which the bridge element is connected to a thermally conductive connector element.
A resin of minimum rigidity and high heat conductivity can be used for the matrix material. For example, doped resin systems with carbon modifications such as carbon nanotubes, graphite or graphene, are suitable. The CFK layers are plate-shaped or film-shaped, for example.
A thermal bridge element such as this has high heat conductivity, with low density and mechanical flexibility of the component at the same time. By embedding the carbon fibers in a matrix material, no impurities can materialize due to a breakage of some of the carbon fibers. As a result of the metallization and the heat-conductive connector elements, which in particular are made of a conventional metal, an essentially loss-free input or discharge of heat into or from the thermal bridge element, and more precisely, into or onto the carbon fibers of the CFK layers can take place. By metallization of the exposed segments of the carbon fibers, a good heat-conductive connection to a respective thermally conductive connector element is possible.
According to an advantageous embodiment, the exposed segments of the carbon fibers are end segments of the carbon fibers. The carbon fibers thereby extend unidirectionally, in a direction from a first of the end segments to a second of the end segments. Ideally, the plate-like or film-like elements have a thickness of 0.15 mm. The result thereof, apart from the good heat conductivity of the bridge element, is also a high flexibility of the bridge element about the axis transversely to the direction of the fibers.
According to a further embodiment, the thickness of the exposed segments in each CFK layer is between 50% and 70%, in particular about 60%, of the thickness of a non-exposed segment of the respective CFK layer.
According to a further embodiment, the fiber volume content of a non-exposed segment of a respective CFK layer is between 50% and 70%, in particular about 60%. This means that in the area of the exposed segments, the bridge element is entirely, or almost entirely, free of the material of the matrix.
According to a further practical embodiment, the metallization on the exposed segments has a thickness of about 5 μm to 20 μm. By providing metallization on the exposed segments, a good heat connection to the respective connector elements can be achieved.
According to a beneficial embodiment, the metallization is formed of copper. In principle, however, other metal materials, for example, gold, silver, and alloys thereof, are also feasible.
In order to achieve a particularly good thermal transition between the metallization and the carbon fibers of the CFK layer(s), the metallization can encompass the individual carbon fibers in the exposed segments. A further improved heat transition comes about due to the fact that the carbon fibers surrounded by the metallization are connected to each other in a material-fit manner, at least in one CFK layer. In particular, in the area of the end segments, the entire layer packet can be thermally conductively connected to one another in a material-fit manner.
It is expedient for a further optimized heat introduction into the carbon fibers, if the metallization borders on end faces of the carbon fibers that are freed of the matrix.
It is preferred that in fiber direction, the carbon fibers have high heat conductivity. For example, a carbon fiber material manufactured by Mitsubishi known under the name K13D2U, which with about 800 W/mK has one of the highest heat conductivities of a carbon fiber on the global market, can be used.
Furthermore, each of the connector elements can be connected in a material-fit manner to the associated metallized segments. With this embodiment, a particularly good heat transfer from the connector elements onto the metallized, exposed carbon fibers occurs. In the case of a purely force-fit connection, in particular by means of clamping, the connector elements can be detached from the layer packets provided with metallization without causing damage.
Furthermore, a method for producing a thermal bridge element of the type as described above is proposed, the method comprising the following steps:                a) Providing a number of CFK layers, wherein each of the CFK layers is composed of a plurality of heat-conductive carbon fibers embedded in a matrix;        b) removing the material of the matrix in at least two segments, particularly opposing end segments, of each of the number of CFK layers so that in the segments, the carbon fibers of a respective CFK layer are exposed;        c) metallizing the exposed segments;        d) stacking the segments of the number of CFK layers provided with a metallization into a layer packet, and establishing in a material-fit manner a connection of the metallized segments of at least some of the number of CFK layers;        e) connecting the metallized segments to an associated thermally conductive connector element.        
The steps b), c), and d) in particular do not need to be carried out in the order specified (b-c-d), but can also be carried out in an order deviating therefrom. For example, steps c) and d) can be transposed with respect to their sequence. Likewise, step d), according to which the stacking of the number of CFK layers into a layer packet takes place, can be carried out prior to step b).
Optionally, the removal of the material of the matrix according to step b) can be done by chemical dissolution of by thermal dissolution. For chemical dissolution, a hydrochloric acid solution can be used, for example, in order to free the specified segments, in particular end segments, of a respective CFK layer from the matrix material. A thermal dissolution can be done by means of laser, for example.
According to an advantageous embodiment, the metallization of the exposed segments according to step c) can be done by galvanization, particularly with copper.
It is beneficial if the material-fit connection according to step d) is established by soldering. This reliably connects the metallized segments of the carbon fibers of a respective layer, as well as different CFK layers, to one another. Moreover, by controlling the quantity of the applied or introduced solder, a secure heat transfer to the thermally conductive connector element as a result of the greatly enlarged contact surface or the thermally active volume can be ensured.
According to a further embodiment of the method, the connector element preferably made of metal is connected to the associated metallized end segment by means of material fit, particularly by soldering; possibly in combination with a connection by force fit. Due to the fact that the force fit is done to a solder having ductile properties, the threat of damage to the carbon fibers as a result of the force introduced by the connector element is greatly reduced or even totally eliminated.
According to a further beneficial embodiment, the generating of the number of CFK layers with a unidirectional orientation of the fibers of the carbon fibers, and the impregnation with a material of the matrix is preferably done using the fiber winding method with a winding tool.